Group A Streptococcus (GAS) is a preeminent pathogen causing a wide spectrum of human diseases. The propensity of particular GAS strains to produce systemic infection defines its capacity to resist host innate immune clearance mechanisms. A clone of the GAS M1T1 serotype spread globally over the last 40 years as the leading cause of invasive infections. Our lab adopted a multifaceted approach to understanding GAS and host factors explaining diverse outcomes of this host-pathogen interaction, using invasive M1T1 GAS clone as a primary model. Our approach coupled precise, targeted of candidate virulence factor genes with in vitro, ex vivo and in vivo models of disease pathogenesis, including WT and knockout mouse lines. We hypothesized that the outcome of GAS infection is dictated by the action and regulation of these GAS virulence factors in response to selective pressures exerted by host innate immunity. In the first funding period, we examined more than two dozen individual virulence factors of the GAS M1T1 clone and their extra- and intracellular interaction with host neutrophils and macrophages, epithelial and endothelial barriers, and soluble immune effectors including antimicrobial peptides, complement proteins and antibodies. For this first renewal application, a key (originally unanticipated) discovery of the past year serves as the central focus for new investigation: our identification of the long-sought-after molecular genetic basis of the hallmark, species-defining, Lancefield group A cell wall carbohydrate antigen (GAC). Our genetic knowledge allowed us to generate the first, precise isogenic GAS M1T1 mutant lacking the Lancefield epitope, a GlcNAc side chain that extends from the polyrhamnose backbone of the antigen. Previously thought only to play a structural role in cell wall biogenesis, we demonstrated that the GlcNAc side-chain contributes to GAS disease pathogenesis by promoting GAS resistance to cationic defense peptides, serum and neutrophil killing, and a mutant lacking the GlcNAC side chain was markedly attenuated for virulence in vivo. The GlcNAc side-chain of GAC has been implicated in the immunopathogenesis of rheumatic heart disease, and we now have a genetic strategy for its elimination. Here we pursue a comprehensive analysis of the GAC GlcNAc side-chain in colonization and systemic virulence of GAS from different disease-associated serotypes, informed by the enhanced mechanistic understanding provided by studies of the original funding period. We will examine the role of the GAC GlcNAc side chain in GAS epithelial cell adherence and invasion, biofilm formation, intracellular survival, interaction with the autophagy system, resistance to macrophage killing, and modulation of leukocyte activation. In vivo, we will assess its role in GAS colonization, necrotizing skin infection and septicemia. Efficacy and safety of GlcNAc- deficient GAC as a universal vaccine antigen will be examined upon conjugation with three different proteins using an active immunization regimen in three different mouse models of GAS disease, with attention to antibody titers, promotion of opsono-phagocytosis and elimination of human tissue cross-reactivity.